Question: On the first 3 tests of his geometry class, Ben got an average score of 81. What does he need on the next test to have an overall average of 85?
Solution: Let his score on the next test be $x$ The sum of all of his scores is then $3 \cdot 81 + x$ The same sum must also be equal to $4 \cdot 85$ Solve: $x = 4 \cdot 85 - 3 \cdot 81 = 97$.